Jewel of Blindness

(Taken from Lawrence Watt-Evans book, The Lure of the Basilisk)
The Jewel of Blindness is a small, dark blue gem set in silver band inscribed with mystic symbols. When activated by the command word, up to 12 people can touch it and become Invisible as the spell. The effect lasts until the person who activated the jewel touches it again, someone attacks or 12 hours pass. Everyone made invisible by the jewel is revealed whenever the effect is broken by anyone in the group.